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208 Posts
Discussion Starter · #1 ·
A farming couple in Northern Michigan consult their physician about severe skin rashes and ulcers noted over the past year. They also have lost many cattle over the past year, and claim that their cattle feed changed in consistency and smell about 1 year ago. Chemical analysis of the feed shows high concentrations of polychlorinated biphenyls, a fertilizer related to known carcinogens. The physician sends the chemical to a laboratory for carcinogen testing, which is performed initially and rapidly by

a. Inoculation of the chemical into nude mice
b. Incubation of mutant bacteria with the chemical to measure the rate of reverse or “back” mutations
c. Incubation with stimulated white blood cells to measure the impact on DNA replication
d. Computer modeling based on the structures of related carcinogens
e. Incubation with mammalian cell cultures to measure the rates of malignant transformation DNA Structure, Replication, and Repair

208 Posts
Discussion Starter · #6 ·
cngrts guys..

The answer is b.
The Ames test is a rapid and relatively inexpensive
bacterial assay for determining mutagenicity of potential toxic chemicals.
Since many chemical carcinogens are mutagenic, it seems obvious that
damage to DNA is a central event in carcinogenesis as well as mutagenesis.
Dr. Bruce Ames developed a tester strain of Salmonella that has been mod-
26 Biochemistry and Genetics
ified not to grow in the absence of histidine because of a mutation in one of
the genes for the biosynthesis of histidine. Toxic chemicals that are mutagens
are placed in the center of the plate and result in reversions of the
original mutations, so that histidine is synthesized and the mutated revertants
multiply in histidine-free media. Since many carcinogens are converted
to active forms by metabolism in the liver, preliminary incubation
with liver homogenates may precede the bacterial assay. Essentially all
chemicals known as carcinogens in humans cause mutagenesis in the Ames
test. The other options-carcinogenicity screening in immunosuppressed
(nude) mice, computer modeling, or incubation with mammalian cell cultures-
may provide some information, but are less efficient and validated
than the Ames test. Contamination of Michigan cattle feed with polychlorinated
biphenyls (PCBs) did occur through an industrial mistake.
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